1. Field of the Invention
The present invention relates to a device on which a backlight is mounted. More specifically, the present invention relates to a backlight unit which suppresses luminance deterioration caused due to changes over time and to a liquid crystal display device using the same.
2. Description of the Related Art
Recently, as display devices for monitors used in mobile terminals such as mobile phones, television sets, and PCs, widely used are liquid crystal display devices which exhibit advantages of being thin-type, light in weight, and low in power consumption.
The luminance of the liquid crystal display device decreases in accordance with the changes over time. In general, it is defined that the luminance life of the liquid crystal display device has run out when the luminance decreases down to 50% of the initial state. The luminance life mainly depends on the luminance life of the element (the light source element) used as the light source such as a cold cathode fluorescent lamp (CCFL) or a light emitting diode (LED) and the durability of the optical members such as the light guide plate, the optical sheet, the reflection sheet, and the like.
As a technique for preventing such changes in the luminance, there are methods for keeping the distance constant by fixing the light source and the light guide plate. Such methods are disclosed in International Patent Application Publication 2010-82377 (Patent Document 1), Japanese Unexamined Patent Publication 2011-65872 (Patent Document 2), and Japanese Unexamined Patent Publication 2010-129376 (Patent Document 3) depicted below, for example.
The lighting device in Patent Document 1 employs a structure which suppresses changes in the positional relation between the light source and the light incident surface caused due to thermal expansion or thermal shrinkage of the light guide plate in accordance with the changes in the temperature within the backlight device through employing a following mechanism which moves the LED unit by following expansion/shrinkage of the light guide plate.
Patent Document 2 discloses a lighting device which includes: a U-shaped holding member to which a light source is fixed via a light-source substrate; and a light guide body having concaves and protrusions formed partially. Concaves and protrusions are provided on the top and bottom faces of the opening part of the holding member, and those are structured to fit with the concaves and protrusions formed on the light guide body. Therefore, fluctuations in the distance between the light source and the optical conductor fixed to the holding member can be suppressed.
The liquid crystal display device disclosed in Patent Document 3 is structured in such a manner that each of the light guide plate, the reflection sheet, and a plurality of optical sheets provided to the backlight is expanded by heat individually, and the positional relation of each of the structural members is maintained by two holding members. Thus, even in a case where there are large differences in the thermal expansion coefficients of each of the members, deflection of the light guide plate and distortion of each sheet generated due to the temperature increase within the device can be prevented. Therefore, changes in the uniformity of the surface luminance of the backlight unit can be suppressed.
Further, as shown in FIG. 17, the liquid crystal display device disclosed in Japanese Unexamined Patent Publication 2005-91526 (Patent Document 4) employs a structure with which a control unit 400 which inputs signals from light receiving units 310 to 330 attached to a backlight unit 120 controls the luminance to be in a specific value through increasing the power supplied to LEDs 210 to 230 when detecting decrease in the luminance, so as to enable to extend its luminance life. In the liquid crystal display device, the luminance gradually decreases and reaches the end of the luminance life at last when the luminance decrease value caused due to deterioration of the LEDs 210 to 230 and the deterioration in the characteristics of the light guide plate 180 and the optical sheets 140, 160 which guide the light from each of those LEDs to the surfaces of the liquid crystal panel 100 exceeds the luminance increase value that can be controlled by the control unit 400.
However, even when the distance between the light source and the light guide plate is maintained uniform as in the techniques disclosed in Patent Documents 1 to 3, the light source is deteriorated, the light flux of the light source is decreased, and the luminance as the entire device becomes decreased due to the changes over time. Further, there is no technical content disclosed for preventing such issue of having deterioration of the light guide plate, the optical sheet, and the like and the luminance is decreased due to the changes over time. That is, the luminance life in the lighting devices and the like of Patent Documents 1 to 3 depends on the luminance life, of the source element and the durability of the optical members as described above. Thus, the limit of the luminance life is almost set if the members and the use conditions are same, and it cannot be extended.
Further, while the luminance life of the liquid crystal display device can be extended to some extent by employing the technical content disclosed in Patent Document 4, it is necessary to additionally provide the structural members such as the light receiving units 310 to 330 and the control unit 400. Therefore, the cost is increased. Furthermore, since the structure with which the control unit 400 increases the power supplied to the LEDs 210 to 230 for supplementing the decreased luminance is employed, the power consumption of the entire device becomes increased in accordance with changes generated over time.
The present invention is designed to improve the inconveniences of the related techniques described above. Specifically, it is an exemplary object of the present invention to provide a backlight unit which effectively extends the luminance life without increasing the cost and the power consumption and to provide a liquid crystal display device using the same.